In order to achieve good image quality for IR systems having detector matrices, the detectors of the matrix need to be individually calibrated. This is carried out partly in the manufacture of the system and partly during operation. A term which is often used in this context is Non Uniformity Connection, so-called NUC. Depending on the detector technology which is used, for example QWIP, MCT, InSb, the solutions can look somewhat different. Essentially, however, it is a question of correcting for the behaviour of the detectors with regard to amplification and offset. The calibration of IR detectors according to NUC principles is described, inter alia, in the article “Cooled IR detectors calibration analysis and optimization” by Patrice Fillon et al, SPIE ORLANDO 2005, [5784-42], pages 1-12.
Primarily due to the fact that the camera systems are not time-stable, the calibration needs to be repeated also during operation. In order to make an entirely new Non Uniformity Correction, two black-body radiators with a relatively large temperature difference are required in order to calculate amplification factors or gain factors for a so-called gain map.
Often the gain map is generated with external flat black-body radiators, i.e. black-body radiators disposed on the input opening of the imaging device. This is preferably done prior to delivery of the imaging device. Certain imaging devices require, however, that an entirely new NUC can be made internally within the device. This applies, inter alia, to devices which are not sufficiently time-stable in terms of temperature to be able to use gain maps generated prior to delivery of the imaging device. Normally, flat black-body radiators are used, which are heated or cooled to mutual, relatively large temperature differences in order to calculate a new NUC.
The introduction of a plurality of black-body radiators into an imaging device, which have to be cooled and/or heated to suitable temperatures, entails a complex and expensive solution which easily becomes heavy and unwieldy. In certain modern-day imaging devices, it has been chosen, therefore, to only use one radiator internal to the imaging device, and without cooling, to achieve a NUC calibration for the generation of a new offset calculation and offset map.
One problem with imaging devices, such as IR systems and IR cameras, having a radiator without cooling is that temperature rises occur primarily due to heat from cooling machinery and electronics. The heating can amount to more than 15° C. This means that the imaging device must handle IR information which can be 15° C. higher than the maximum working temperature specified for the imaging device. With low maximum values, for example 60° C., for scene temperatures, values for noise equivalent temperature difference, NETD, and thus values for minimum resolvable temperature difference MRTD, suffer when the response must be lowered to ensure that the temperature of the radiator will be able to be used to calculate a new offset map. With, for example, a maximum working temperature of 70° C. and a temperature rise, due to internal warming, of 15° C., the imaging device is necessarily set for a scene temperature of 85° C., which means that the NETD and MRTD are considerably worsened.